The present invention relates to a process for the preparation of acetic acid and/or methyl acetate in the liquid phase in the presence of a homogeneous catalyst comprising iridium and platinum.
The process of the invention makes it possible to obtain an increased productivity in terms of acetic acid, as well as an improved stability of the catalyst system employed.
Various ways of obtaining acetic acid are known and exploited in industry. These include the methanol carbonylation reaction carried out in the liquid phase, under a pressure of carbon monoxide, which is one of the reactants, in the presence of a homogeneous catalyst system. Another way of obtaining acetic acid consists in isomerizing methyl formate. This reaction, too, is generally carried out in the presence of a catalyst system in the homogeneous phase. Finally, another process involves carrying out the carbonylation of methanol and the isomerization of methyl formate simultaneously.
More precisely, the carbonylation process using rhodium is a known process, exploited in industry, which has formed the subject of numerous articles and patents, for example American patents U.S. Pat. No. 3,769,329 and U.S. Pat. No. 3,813,428.
European patents EP 618 183 and EP 618 184, and European patents EP 785 919 and EP 759 022, describe a carbonylation process carried out in the presence of a catalyst system based on iridium and, if appropriate, also containing rhodium.
A carbonylation process using iridium and ruthenium, which is currently exploited in industry, is described in European patent EP 643 034.
More recently, a new preparative process, consisting of a methyl formate isomerization reaction in the presence of iridium, was proposed in French patent FR 2 746 794 and international patent application WO 97/135829.
In parallel, a process for the preparation of acetic acid and/or methyl acetate by carrying out a methyl formate isomerization reaction and a methanol carbonylation reaction simultaneously was proposed in patent FR 2 746 795 and international patent application WO 97/35828.
These different processes for acetic acid production are generally carried out continuously in plants comprising essentially three zones. The first corresponds to the actual reaction zone comprising a pressurized reactor in which the carbonylation and/or isomerization are carried out. The second consists of a zone for separation of the acid formed. This operation is effected by partial vaporization of the reaction mixture in a so-called flash apparatus, in which the pressure is kept below that in the reactor. The vaporized part is then sent to a third zone, where the acetic acid produced is purified. This zone comprises various distillation columns in which the acetic acid produced is separated from the water, the reactants and the by-products. That part of the mixture which remains in liquid form at the outlet of the vaporization zone, and comprises especially the catalyst, is recycled to the reactor.